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White
Paper

Fundamentals, System Design, and Setup for the 4 to 20 mA Current
Loop

Updated
Aug 2, 2019

Overview

Since the 1950s, current loops have been used to transmit
transducer data for process monitoring and control. With their low
implementation cost, inherent resistance to noise, and ability to carry signals
long distances, current loops have proven particularly well-suited for
industrial environments. This white paper focuses on current loop fundamentals,
system design, and setup.

Contents

Designing
a Current Loop System

Current
Loops: Beyond the Basics

Summary

Current Loop Fundamentals

Using Current to Transmit Transducer Data
Industrial-grade
transducers commonly use current signals to transmit data. This is in contrast
to most other transducers, such as thermocouples and resistive strain gages,
which use a voltage signal. While voltage-based transducers are sufficient in
many environments, current-based transducers are at times preferred. For
instance, the inherent disadvantage of using voltage to transmit signals in an
industrial environment is voltage drops over long cable lengths due to wire
resistance. You can use high input impedance devices to circumvent signal loss.
However, these devices are sensitive to the noise induced by nearby motors,
conveyor belts, and radio transmissions.

Kirchhoff’s current law (KCL) states that the sum of currents
flowing toward a point is equal to the sum of currents flowing away from that
point. In theory, all current present at the beginning of a loop must reach the
end as demonstrated in Figure 1.

Figure 1. According to Kirchhoff’s current law, all current
present at the beginning of a loop must reach the end.

This is the basic principle on which a current loop operates. Measuring
current at any position along a single loop produces the same result. By using
current signals and low impedance data acquisition devices, industrial
applications benefit from better noise immunity and longer transmission cable
lengths.

Current Loop Components
The main components of a
current loop include a DC power supply, transducer, a data acquisition device,
and wires connecting them together in a series, as shown in Figure 2.

Figure 2. Current Loop System

The DC power supply provides the power to the system. The transducer
regulates the flow of current through the wires at 4 to 20 mA, where 4 mA
represents a live zero and 20 mA represents the maximum signal. A 0 mA signal
signifies a break in the loop. The data acquisition device measures the
regulated current flow. An effective and accurate method of measuring current is
to place a precision shunt resistor between the leads of the data acquisition
device amplifier (in Figure 2) to convert the current signal into a voltage
measurement to ultimately be related back to the transducer’s original
measurement.

To help better understand current loops, consider the design of an example
system using a transducer throughout this white paper. System specifications
include the following:

Transducer needs to measure pressure

Transducer is located 2000 ft away from the measurement device

A current measuring the data acquisition device provides an operator with
feedback about the transducer measurements

This example begins with determining an appropriate transducer
for the application.

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to top

Designing a Current Loop System

Choosing a Transducer
The first step in designing a current loop
system is selecting a transducer. Beyond the type of measurement (flow,
pressure, temperature, and so on), an important consideration in choosing any
transducer is the operating voltage. A voltage source provides power to the
current-based transducer, allowing it to regulate the flow of current through
the wires. To maintain transducer functionality, the voltage supplied should be
no less than the minimum required; likewise, supplying more voltage than the
maximum specified can damage the transducer.

For the example current loop system, your chosen transducer measures pressure
and has an operating voltage of 12 to 30 V. With the transducer selected,
properly measuring the current signal is required to provide an accurate
representation of the pressure applied to the transducer.

Choosing a Current Measuring Data Acquisition
Device
An important aspect of building a current loop system is avoiding
ground loops. A common method of preventing ground loops with a measuring data
acquisition device is isolation. With isolation, you can avoid ground loops such
as the one in Figure 3.

Figure 3. Ground Loop

Ground loops are created when two connected terminals in a circuit are at
different ground potentials. This difference causes current to flow in the
interconnection, which can produce offset errors.

Isolating the data acquisition device is a means of electrically separating
signal source ground from the amplifier ground of the data acquisition device,
as shown in Figure 4. Because current cannot flow across the isolation barrier,
the amplifier ground reference can be at the same potential as the signal source
ground. Thus, you cannot inadvertently create a ground loop.

Figure 4. Common- and Normal-Mode Voltage with
Isolation

Isolation also prevents large common-mode voltages from damaging data
acquisition devices. Common-mode voltage is the voltage present on both the
positive and negative inputs of an instrumentation amplifier. For example, both
+ and – pins of the data acquisition device in Figure 4 are on top of +14 V of
common-mode voltage. Many data acquisition devices have a maximum input range of
±10 V. If the data acquisition device has no isolation and the common-mode
voltage is outside the maximum input range, you may damage the device. Even
though the normal-mode voltage in Figure 4 is only +2 V with a +14 VDC offset,
the resulting working voltage is +16 V (normal-mode voltage is the voltage
difference between the + and – pins; working voltage is normal- and common-mode
voltage added together). This would be a hazardous voltage level to a data
acquisition device with a smaller working voltage.

With isolation, the amplifier ground reference is electrically separated from
earth ground. As in Figure 4, the amplifier ground reference may be
level-shifted to match the +14 V. Raising the amplifier ground reference to +14
V effectively shifts +16 to +2 V at the data acquisition amplifier. Now the data
acquisition device is no longer at risk of overvoltage damage. (Note that
isolators have a maximum common-mode voltage they can reject.)

With the current measuring data acquisition device isolated and
protected, matching a power supply is the last step in completing a basic
current loop system.

Choosing a Power Supply
Discovering which power supply best meets
your requirements is simple. In a current loop, the power supply must emit
voltage equal to or greater than all the voltage drops in the system
combined.

The data acquisition device for this example uses a precision
shunt resistor to measure current. You need to calculate the voltage drop across
the resistor. A typical shunt resistor is 249 Ω. Implementing basic calculations
with the 4 to 20mA current loop shows the following:

With this 249 Ω shunt resistor, you can map current to a range of 1 to 5 V,
allowing the data acquisition amplifier’s voltage reading to be related to the
pressure transducer’s signal.

The pressure transducer from above requires a minimum operating
voltage of 12 V with a maximum of 30 V. Adding the voltage drop of the precision
shunt resistor to the transducer yields the following:

This suggests that a 17 V power supply suffices. However, the current
loop wire’s internal resistance creates an additional load on the power
supply.

In applications where a transducer is far away from the
measuring instruments, you must factor wire resistance into the loop power
calculations. Copper wires have a DC resistance that is directly proportional to
their lengths and diameters (gauge). With the pressure transducer from the
example system, you need to calculate the 2000 ft distance in the power
requirements. With 24-gauge solid copper wire, the internal resistance is 2.62
Ω/100 ft[1].
This resistance calculation becomes:

At 2000 ft away, there is a 1.05 V drop along the wire. To
complete the loop, you need two wires, effectively doubling the length to 4000
ft:

In total, approximately 2.1 V is lost due to wiring when the
pressure transducer is 2000 ft away from the data acquisition device. Combining
all the voltage drops results in the following:

If you used a 17 V power supply to drive the previous setup, the pressure
transducer voltage would be below the minimum operating level for a transducer
due to natural wire resistance and shunt resistor requirements. Choosing a
common 24 V power supply meets the power requirement of the pressure transducer
system.  If desired, there is enough voltage to place the pressure
transducer at a further distance.

With the transducer, data acquisition device, cable lengths, and
power supply properly matched, the simple pressure transducer loop system is
complete. For larger applications, you can incorporate additional measurements
into the system.

Adding More Transducers and Instruments
For larger
applications, you can add more transducers and instruments in parallel to the
power supply, as shown in Figure 5.

Figure 5. Adding More Transducers

Because you add the extra current loops in parallel, the power supply’s
voltage doesn’t need to change; you only need to make sure it can source the
required current. You can mix and match different types of transducers as long
as each transducer and power supply’s operating voltage match. In the previous
example, as long as the new transducer’s maximum operating voltage is not lower
than the power supply’s 24 V, you can add the new transducer in parallel.

At times, you may need multiple current measuring data
acquisition devices to measure the same signal. To add another data acquisition
device to the same transducer, you must place the new instrument in series with
the particular loop. For this example, you add a simple digital current loop
meter with the existing data acquisition device, as shown in Figure 6.

Figure 6. Adding an Extra Instrument to a
Transducer

Each time you add a current-measuring data acquisition device in series with
the transducer, you must recalculate the total voltage requirement to ensure
that the power requirements of the loop are still within specification. In this
example, the new current meter uses a 105 Ω precision resistor:

With a 105 Ω shunt resistor, the current meter requires a
maximum of 2.1 V, so the total voltage requirement of the system becomes:

Because adding the meter brings the total voltage requirement to only about
21.2 V, the 24 V power supply previously recommended is still sufficient for the
application.

With minor calculations and additional hardware, you can quickly
expand a current loop system’s application base. Also, with newer standards and
types of transducers, you can extend the measurement capabilities of existing
current loop infrastructures.

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to top

Current Loops: Beyond the Basics

Digital signaling developments have broadened the current loop's
application space and delivered more functionality to an already robust,
low-cost system.

The Highway Addressable Remote Transducer (HART), developed by Rosemount
Inc., is a digital signaling protocol that can run concurrently with existing
current loop systems. With digital signaling, HART-enabled transducers allow
responses to proprietary commands to perform certain actions or reply with a
status beyond the 4 to 20 mA current signaling. Using this technology,
industrial automation and control applications can actively monitor and respond
to situations by communicating between intelligent field instruments and host
systems.

Learn more about HART.

For specialty applications such as magnetic flowmeters, 3-wire (Type 3) and
4-wire (Type 4) transducers are also available.

Type 3 transducers provide two transducers packaged in one. These transducers
are designed and installed in the same manner as the 2-wire transducers
explained in this white paper.

Type 4 transducers provide separate connections for power and current
measurements. Because 4-wire transducers decouple power from the measurement
system, the need for an isolation requirement at the measuring data acquisition
device is lifted. A drawback of 4-wire transducers, though, is that the power
source requires a separate pair of wires that must run the cabling distance.
This causes many applications to place the power source near the
transducers.

Learn more about Type 3 and Type 4 transducers.

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Summary

Current loops are ideal for industrial environments
where long cable lengths are required or large amounts of electromagnetic
interference exist between the transducer and data acquisition
device. Whether automating a factory floor or taking field measurements,
you can find a variety of National Instruments isolated current measuring
devices across different platforms to meet your application needs.

Current Measurement Products from National Instruments

NI CompactRIO – Reconfigurable Embedded Control and
Acquisition System

NI
9203 – 16-Bit Analog Current Input Module

NI
9219 – Isolated Universal Analog Input Module

NI
CompactDAQ –
Modular Data Acquisition System

NI
9203 – 16-Bit Analog Current Input Module

NI
9219 – Isolated Universal Analog Input Module

NI
Industrial M Series

NI
PXI-6239 – 8 Analog
Current Inputs, 2 Analog Current Outputs, Bank Isolation

NI
PCI-6238 – 8 Analog
Current Inputs, 2 Analog Current Outputs, Bank
Isolation

Related Links

Isolation
Technologies for Reliable Industrial Measurements

How
to Minimize Errors for Low-Current Measurements

DC
and AC Current Measurements

References
[1]
http://www.radiolocman.com/shem/shem-cache.html?di=18899

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